Method for producing aluminum titanate sintered object

ABSTRACT

The present invention provides a process for preparing an aluminum-titanate-based sintered body comprising the step of firing, at 1250 to 1700° C., a formed product prepared from a raw material mixture containing 100 parts by weight of a mixture of TiO 2  and Al 2 O 3  in a weight ratio of TiO 2 :Al 2 O 3 =40:60 to 60:40, and 1 to 15 parts by weight of an alkali feldspar represented by the formula:(Na x K 1-x )AlSi 3 O 8  wherein 0≦x≦1. 
     According to the process of the present invention, it is possible to obtain an aluminum-titanate-based sintered body in which inherent properties of an aluminum titanate, i.e., a low coefficient of thermal expansion and high corrosion resistance are maintained, the mechanical strength thereof is improved, and which can be stably used even under high temperature conditions.

TECHNICAL FIELD

The present invention relates to a process for preparing analuminum-titanate-based sintered body.

BACKGROUND ART

A sintered body of aluminum titanate has a low thermal expansioncoefficient and high corrosion resistance, and is known as aheat-resistant material which exhibits low wettability with moltenmetal, corrosion resistance, spalling resistance and other excellentproperties when used as a material for container, ladle, gutter, etc.,for molten metal of aluminum, aluminum alloys, pig iron or the like.However, since the crystal grains constituting a sintered body ofaluminum titanate are anisotropic, the sintered body has drawbacks thatthe micro crack formation at the crystal grain boundary often occurs dueto the stress caused by thermal expansion, and that the mechanicalstrength is easily weakened pursuant to the progress of micro cracks andgaps.

Thus, a conventional sintered body of aluminum titanate is insufficientin strength, and, particularly, can not exhibit sufficient durabilitywhen used under high temperature and loaded conditions.

Further, since aluminum titanate is unstable at temperatures of 1280° C.or below and easily decomposed into TiO₂ and Al₂O₃ when used under hightemperature conditions of approximately 800 to 1280° C., it is difficultto continuously use the sintered body of aluminum titanate within suchtemperature range.

In order to improve sinterability of aluminum titanate and restrain thethermal decomposition thereof, additives such as silicon dioxide aremixed with the raw material for sintering. In this case, however, therefractoriness of the obtained sintered body is easily diminished, andit is not possible to obtain a sintered body of aluminum titanate havingrefractoriness so as to be usable in high temperatures over about 1400°C. and also possessing high mechanical strength.

DISCLOSURE OF INVENTION

A primary object of the present invention is to provide analuminum-titanate-based sintered body in which inherent properties of asintered body of aluminum titanate, i.e., low thermal expansioncoefficient and superior corrosion resistance are maintained, themechanical strength thereof is improved, and which can be stably usedeven under high temperature conditions.

In view of the foregoing problems of the prior art, the presentinventors carried out extensive research. Consequently, the inventorsfound that when producing a sintered body of aluminum titanate bysintering a raw material powder containing titanium dioxide and alumina,Si atoms dissolve in the aluminum titanate crystals by adding a specificalkali feldspar to the raw material powder and the growth of the crystalgrain is restrained to give a dense sintered body. Furthermore, theinventors found that the obtained sintered body possesses both highmechanical strength and low thermal expansion coefficient and, inaddition, is superior in decomposition resistance, refractoriness, andso on. The present invention has been accomplished based on thesefindings.

The present invention provides a process for preparing analuminum-titanate-based sintered body as described below.

-   1. A process for preparing an aluminum-titanate-based sintered body    comprising the step of firing a formed product at 1250 to 1700° C.,    -   the formed product being prepared from a raw material mixture        containing:    -   100 parts by weight of a mixture containing TiO₂ and Al₂O₃ in a        weight ratio of TiO₂:Al₂O₃=40:60 to 60:40, and    -   1 to 15 parts by weight of an alkali feldspar represented by the        formula: (Na_(x)K_(1-x))AlSi₃O₈ wherein 0≦x≦1.-   2. The process for preparing an aluminum-titanate-based sintered    body according to item 1, wherein the formed product is fired under    a reducing atmosphere.-   3. The process for preparing an aluminum-titanate-based sintered    body according to item 1 or 2, wherein x in the formula:    (Na_(x)K_(1-x))AlSi₃O₈ is 0.1≦x≦1.-   4. An aluminum-titanate-based sintered body obtainable by the    process according to any one of items 1 to 3 above.

The process for preparing an aluminum-titanate-based sintered bodyaccording to the present invention is a method wherein a formed productis prepared from a raw material obtained by adding an alkali feldsparrepresented by the formula: (Na_(x)K_(1-x)) AlSi₃O₈ (0≦x≦1) to a mixturecontaining TiO₂ and Al₂O₃, and fired at a temperature ranging from 1250to 1700° C.

Usable TiO₂ and Al₂O₃ as the raw material are not limited insofar asthey are capable of forming aluminum titanate when fired. Normally, theyare suitably selected from the raw materials for producing variousceramics such as alumina ceramics, titania ceramics, aluminum titanateceramics, and so on. Particularly, when using anatase TiO₂ as TiO₂, andsinterable alpha-alumina as Al₂O₃, the reactivity of both constituentsis high, and it is possible to form aluminum titanate in a short periodof time and in high yield.

The mixing ratio of TiO₂ and Al₂O₃ may be in a range wherein TiO₂:Al₂O₃(weight ratio)=about 40:60 to about 60:40, and preferably in a rangewherein TiO₂:Al₂O₃ (weight ratio)=about 40:60 to about 45:55.

The alkali feldspar used as the additive is represented by the formula:(Na_(x)K_(1-x)) AlSi₃O₈, wherein x in the formula is 0≦x≦1.Particularly, in the aforementioned formula, the range of 0.1≦x≦1 ispreferable, and the range of 0.15≦x≦0.85 is more preferable. The alkalifeldspar having an x value in the range as described above has a meltingpoint lower than the formation temperature of aluminum titanate, and isespecially effective in promoting the sintering of aluminum titanate.

The amount of alkali feldspar to be used may be about 1 to about 15parts by weight, preferably about 4 to about 10 parts by weight based on100 parts by weight of the total weight of TiO₂ and Al₂O₃.

According to the process of the present invention, by mixing theaforementioned specific alkali feldspar as an additive with the mixturecontaining TiO₂ and Al₂O₃, forming the mixture into a desired shape andthen firing the formed product, the grain growth of aluminum titanate isrestrained and a dense sintered body can be obtained. The reason forthis is believed to be that when synthesizing aluminum titanate byfiring, Si within the alkali feldspar dissolves in the crystal latticeand is substituted for Al, and, as Si has a smaller ion radius than Al,the bond length with the surrounding oxygen atoms is shortened, and thecrystal is densified as a result thereof.

The raw material mixture comprising TiO₂, Al₂O₃ and alkali feldspar maybe sufficiently mixed, pulverized to a suitable particle size, andformed into a desired shape.

There is no particular limitation on the method of mixing andpulverizing the raw material mixture, and an ordinary method may beemployed; for example, mixing and pulverization may be conducted withthe use of a ball mill, media agitating mill, or the like.

There is no particular limitation on the degree of pulverizing the rawmaterial mixture, but the raw material mixture is preferably pulverizeduntil a grain size of approximately 1 μm or less is attained.

When necessary, it is also possible to mix a forming aid with the rawmaterial mixture. The forming aid for use herein may be selected fromthose which have been conventionally used depending on the formingmethod.

Such useful forming aids include binders such as polyvinyl alcohol,microwax emulsion and carboxymethyl cellulose; mold releasing agentssuch as stearic acid emulsion; antifoaming agents such as n-octylalcohol and octylphenoxy ethanol; and deflocculating agents such asdiethylamine, triethylamine, etc.

There is also no particular limitation on the amount of such formingaids to be used, and the amount may be suitably selected within therange of the amount of conventional forming aids depending on theforming method. For example, in slip casting, the binder may be used inan amount of about 0.4 to about 0.6 parts by weight; the deflocculatingagent may be used in an amount of about 0.5 to about 1.5 parts byweight; the mold releasing agent may be used in an amount of about 0.2to about 0.7 parts by weight (solid weight); and the antifoaming agentmay be used in an amount of about 0.03 to about 0.1 parts by weight, allbased on 100 parts by weight of the total weight of TiO₂ and Al₂O₃.

There is also no particular limitation on the method of forming the rawmaterial mixture and conventional forming methods such as press molding,sheet casting, slip casting, extrusion molding, injection molding, CIPmolding, etc. may be suitably employed.

The firing temperature may be about 1250 to about 1700° C., preferablyabout 1400 to about 1700° C. There is no particular limitation on theatmosphere for firing, and any one among oxygen-containing atmospheresuch as air, reducing atmosphere, inactive atmosphere, etc.conventionally employed maybe used. Particularly, firing under reducingatmospheres such as hydrogen atmosphere, carbon monoxide atmosphere,natural gas atmosphere and LPG atmosphere is effective, as a densesintered body with superior strength can be formed easily.

There is no particular limitation on the firing time, and firing may becontinued until the sintering reaction sufficiently progresses inaccordance with the shape of the formed product, etc., and, normally,firing is conducted for about 1 to about 10 hours while maintaining theaforementioned temperature range. There is also no particular limitationon the heating rate and cooling rate upon firing insofar as such ratesare set to conditions where cracks will not be generated in the sinteredbody.

The sintered body obtained by the process of the present invention is analuminum-titanate-based sintered body wherein Si dissolves in thecrystal lattice of aluminum titanate and is substituted for Al, and thelattice constant has a smaller value in comparison with pure aluminumtitanate. As a result, the obtained sintered body has a stable crystalstructure and becomes a sintered body with fine crystal grains becausethe crystal grain growth is restrained. The sintered body as describedabove in which the crystal grain growth is restrained does not need togenerate cracks aiming at relaxing the distortion caused by thermalexpansion, and becomes a dense product with high mechanical strength.

The sintered body obtained by the process of the present inventionpossesses superior characteristics as described above; for example, highmechanical strength and low thermal expansion coefficient. In addition,as the crystal structure is stabilized, this sintered body is alsosuperior in decomposition resistance and refractoriness. As a result,this sintered body can be stably used under temperatures from severalhundred degrees Celsius to about 1600° C. as the decomposition reactionof aluminum titanate is restrained. Moreover, the sintered bodypossesses refractoriness of SK 40 (1920° C.) or more which far exceedsthe melting point of aluminum titanate, which is 1860° C. Further, thesintered body obtained by the process of the present invention hasextremely superior non-wetting property and corrosion resistance againstmolten metal, and, as a result, exhibits superior erosion resistanceinconceivable with conventional material against flowing molten metal.

By utilizing the superior characteristics described above, thealuminum-titanate-based sintered body of the present invention may beused for various purposes; for example, containers for high meltingpoint metals such as crucibles, ladles, and gutters; high temperaturecomponents of aircraft jet engines; jet nozzles; high temperaturecomponents of various internal combustion engines such as glow plugs,cylinders and piston heads; outer wall thermal insulation and shields ofspace crafts; and so on. Moreover, by utilizing its low expansioncharacteristics, this aluminum-titanate-based sintered body may also beeffectively used as a surface plate for printing processing in an LSImanufacturing process.

As described above, the aluminum-titanate-based sintered body obtainedby the process according to the present invention maintains a lowcoefficient of thermal expansion which is an inherent property ofaluminum titanate, possesses high mechanical strength, and has highthermal shock resistance. Moreover, this aluminum-titanate-basedsintered body has extremely high refractoriness of SK 40 (1920° C.) ormore which is prescribed in JIS R 2204, exhibits superior decompositionresistance, and can be stably used under high temperature conditions.

EXAMPLES

The present invention is now explained in detail with reference to thefollowing examples.

Example 1

To a mixture (100 parts by weight) consisting of 43.9% by weight oftitanium oxide in anatase form and 56.1% by weight of sinterablealpha-alumina, 4 parts by weight of Fukushima-grown alkali feldspar((Na_(0.39) K_(0.61))AlSi₃O₈) as the additive, 1.5 parts by weight ofdiethanolamine as the deflocculating agent, 0.4 parts by weight ofpolyvinyl alcohol as the binder, and 30 parts by weight of water wereadded to obtain the raw material mixture. This raw material mixture wasplaced in a ball mill and mixed for 3 hours. The obtained slurry wasthen left to stand for 50 hours, thereafter cast in a crucible mold, andremoved after 2 hours to obtain a cylindrical compact with a 6 cmdiameter and 8 cm height.

After air-drying this compact for 24 hours, the compact was furtherdried in a drier at a temperature of 60° C. or below until the watercontent became 1% or less.

After removing the compact from the drier, the compact was heated to1600° C. in 13 hours, fired at 1600° C. for 2 hours, and thereafter leftto cool. Firing was conducted under atmospheric atmosphere.

The lattice constant calculated from the X-ray diffraction patternregarding the obtained sintered body is shown below in Table 1. Thelattice constant of a pure aluminum titanate is also shown forcomparison.

TABLE 1 Lattice Constant (Å) a b C True Density Sintered Body of Example1 9.423 9.626 3.586 3.713 Aluminum Titanate 9.429 9.636 3.590 3.704

As is clear from the above results, with the sintered body obtained bythe process of the present invention, it has been confirmed that thelattice constant is smaller than aluminum titanate in all crystal axes,and that Si was dissolved in aluminum titanate crystal by substitution.

Further, Table 2 shows the results upon measuring the thermal expansioncoefficient of this sintered body and pure aluminum titanate at aheating rate of 20° C./minute, and Table 3 shows the results uponmeasuring the thermal expansion coefficient of this sintered body andpure aluminum titanate at a cooling rate of 20° C./minute.

TABLE 2 Thermal Expansion Coefficient (%) (Heating) Temperature SinteredBody ° C. of Example 1 Aluminum Titanate 30 0 0 80 −0.007 −0.005 130−0.016 −0.011 180 −0.023 −0.019 230 −0.03 −0.027 280 −0.036 −0.034 330−0.04 −0.04 380 −0.043 −0.044 430 −0.045 −0.048 480 −0.045 −0.049 530−0.044 −0.048 580 −0.041 −0.046 630 −0.037 −0.042 680 −0.03 −0.037 730−0.02 −0.032 780 −0.006 −0.028 830 0.008 −0.026 880 0.022 −0.023 9300.039 −0.017 980 0.056 −0.012 1000 0.063 −0.011

TABLE 3 Thermal Expansion Coefficient (%) (Cooling) Temperature SinteredBody ° C. of Example 1 Aluminum Titanate 1000 0.019 −0.072 980 0.004−0.085 930 −0.029 −0.112 880 −0.063 −0.131 830 −0.099 −0.146 780 −0.137−0.159 730 −0.174 −0.173 680 −0.21 −0.187 630 −0.241 −0.192 580 −0.246−0.184 530 −0.233 −0.17 480 −0.216 −0.155 430 −0.196 −0.138 380 −0.177−0.121 330 −0.155 −0.101 280 −0.132 −0.081 230 −0.108 −0.06 180 −0.082−0.037 130 −0.056 −0.011 80 −0.027 0.018 50 −0.003 0.038

As is clear from the results indicated above, the sintered body obtainedby the aforementioned method has a small thermal expansion coefficient,and maintains the low expansion characteristics inherent in aluminumtitanate.

Further, as a thermal shock resistance test on the sintered bodyobtained in Example 1, a rapid cooling test by placing the sintered bodyheated to 1250° C. into ice water of 0° C. as well as a rapid heatingtest by rapidly heating a sintered body of −25° C. to 1500° C. with agas burner were conducted. The result was that no cracks were generated,and the sintered body showed superior thermal shock resistance.

Example 2

The same raw material mixture used in Example 1 was mixed in a ball millfor 3 hours, and the obtained slurry was dried at 120° C. for 4 hours,and thereafter molded into a shape of 120 mm×35 mm×25 mm (thickness) or120 mm×35 mm×20 mm (thickness) (specimen for measuring a coefficient ofliner expansion on heating) at a molding pressure of 60 MPa.

The obtained compact was fired with the following firing pattern 1 orfiring pattern 2 and left to cool to obtain an aluminum-titanate-basedsintered body.

-   1. Firing Pattern 1 (Firing at 1540° C.)    -   from 0 to 180° C. in 4 hours    -   from 180 to 250° C. in 3 hours    -   from 250 to 450° C. in 3 hours    -   at 450° C. for 3 hours    -   from 450 to 1540° C. in 6 hours    -   at 1540 ° C. for 2 hours-   2. Firing Pattern 2 (Firing at 1600° C.)    -   from 0 to 180° C. in 4 hours    -   from 180 to 250 ° C. in 3 hours    -   from 250 to 450° C. in 3 hours    -   at 450° C. for 3 hours    -   from 450 to 1600° C. in 6 hours    -   at 1600° C. for 2 hours

Measurement results of the physical properties of the respectivesintered bodies obtained above are shown below in Table 4.

TABLE 4 Firing Temperature (° C.) 1540 1600 Contraction Rate by Firing(%) −9.63 −9.55 Apparent Porosity (%) 7.3 5.5 Water Absorption (%) 2.21.7 Apparent Specific Gravity 3.56 3.49 Bulk Specific Gravity 3.30 3.30Refractoriness (SK) 40 or more 40 or more Bending Strength (MPa) 50 40Room Temperature Liner Expansion Rate on Heating (%) 500° C. −0.09 ±0750° C. −0.08 −0.02 100° C. +0.04 +0.10

As is clear from the results indicated above, the sintered bodiesobtained by the aforementioned method have a low thermal expansioncoefficient, and possess high refractoriness and high mechanicalstrength.

The thermal shock resistance test was conducted on the sintered bodiesobtained in Example 2 by the same manner as in Example 1. The result wasthat no cracks were generated, and the sintered bodies showed superiorthermal shock resistance.

1. A process for preparing an aluminum-titanate-based sintered bodycomprising the step of firing a formed product at 1250 to 1700° C., theformed product being prepared from a raw material mixture containing:100 parts by weight of a mixture containing TiO₂ and Al₂O₃ in a weightratio of TiO₂: Al₂O₃=40:60 to 60:40, and 1 to 15 parts by weight of analkali feldspar represented by the formula: (Na_(x)K_(1-x))AlSi₃O₈wherein 0≦x≦1.
 2. The process for preparing an aluminum-titanate-basedsintered body according to claim 1, wherein the formed product is firedunder a reducing atmosphere.
 3. The process for preparing analuminum-titanate-based sintered body according to claim 1 or 2, whereinx in the formula: (Na_(x)K_(1-x))AlSi₃O₈ is 0.1≦x≦1.